Bound vmcp health and session registry growth

[lorenzozanee]

Summary

All VirtualMCPServer instances grow 2-3x RSS over ~24h, scaling with backend count (12/13/14/19 backends grew 2.0-3.1x vs flat control proxies). Investigation shows two unbounded in-memory maps:

• health.statusTracker.removedBackends kept a tombstone per removed backend forever to avoid in-flight health-check races. Under ID churn it grew linearly without expiry.
• server.healthResyncRegistry kept a per-session listChangedResyncWorker (holding ClientSession + identity) for sessions that ended via TTL expiry. Pruning only happened on the next async resync, so the registry stayed larger than the live session set between health-change events.

What changed:

• Bound removedBackends with a TTL of 2*checkInterval (default 60s, 5m fallback). Entries expire lazily on isRemoved and are pruned periodically in UpdateBackends and performHealthCheck, and proactively at RemoveBackend insertion.
• Filter dead sessions synchronously at health-change fan-out: snapshot now returns a map, resyncSessionsOnBackendHealthChange checks GetMultiSession for each entry and removes dead sessions before triggering, keeping |workers| == |liveSessions|.

Fixes #5860

Type of change

• Bug fix
• New feature
• Refactoring (no behavior change)
• Dependency update
• Documentation
• Other (describe):

Test plan

• Unit tests (task test)
• E2E tests (task test-e2e)
• Linting (task lint-fix)
• Manual testing (describe below)

New regression tests:

• pkg/vmcp/server/regression_5860_health_resync_test.go: verifies fan-out prunes dead sessions and only triggers live sessions (fails on base with 4 workers vs 2 live).
• pkg/vmcp/health/regression_5860_tombstone_test.go: churns 50 distinct backends and asserts tombstones are bounded after TTL (fails on base with 50 entries).

Existing pkg/vmcp/health and pkg/vmcp/server suites pass (task test with -race).

API Compatibility

• This PR does not break the v1beta1 API, OR the api-break-allowed label is applied and the migration guidance is described above.

Changes

File	Change
pkg/vmcp/health/status.go	map[string]bool -> map[string]time.Time with TTL, lazy and periodic prune
pkg/vmcp/health/monitor.go	Inject checkInterval, prune in UpdateBackends/performHealthCheck
pkg/vmcp/server/serve_health_resync.go	snapshot map, synchronous GetMultiSession filter at fan-out
pkg/vmcp/server/serve_health_resync_test.go	Adapt coalescing test to new filter path
pkg/vmcp/health/regression_5860_tombstone_test.go	New bounded-tombstone regression
pkg/vmcp/server/regression_5860_health_resync_test.go	New bounded-registry regression